I. Field of the Invention
This invention relates generally to implantable cardiac stimulating apparatus, and more particularly to a rate adaptive cardiac rhythm management device having a sensor for measuring trans-thoracic impedance variations and for deriving therefrom a control signal varying as a function of the patient's minute ventilation and which uses that control signal to adjust the rate at which it generates cardiac stimulating pulses to thereby accommodate the patient's physiologic demand.
II. Discussion of the Prior Art
The prior art is replete with patents which are designed to accommodate a patient's physiologic demand. In fact, there has been developed a whole class of implantable cardiac pacemakers which are referred to as being "rate adaptive", meaning that they do not merely generate cardiac stimulating pulses at a fixed, programmed rate irrespective of the patient's level of activity, but instead, operate to sense some parameter that correlates with metabolic need and then using the sensed signal, deriving a rate-controlling index for adjusting the pacing rate between a minimum or lower rate limit and a maximum or upper rate limit.
Parameters that have been sensed in the past include blood Ph, blood temperature, QT interval, blood oxygen saturation, respiratory rate, minute ventilation, etc. Specific control algorithms for developing the control signal based upon variations of the above parameters have each met with some problems. For example, where blood Ph is the parameter, sensor stability has been a problem. The Ph sensors used have been found to age and drift with time. Blood oxygen saturation can be measured using light emitters and sensors but they tend to complicate the lead system used to couple the pacemaker's pulse generator to the heart. Body temperature, on the other hand, while readily measured with a thermistor-type device, has proven to be a poor choice for use in a rate adaptive pacemaker because of the lag between the start of exercise and the detectable increase in blood temperature. Similarly, the use of ECG-based information, such as the pre-ejection period or the QT interval have not been entirely successful because of the difficulty in detecting the parameters in the presence of other myopotentials and motion artifacts typically involved.
Respiratory rate is also not a particularly suitable parameter on which the derivation of a rate control signal can be based. This is due to the fact that the correlation between heart rate and respiratory rate is not particularly strong or linear.
Another respiratory-related parameter that has been found to closely correlate with physical exercise and, therefore, metabolic demand is minute ventilation which may be defined as the volume of air inspired and expired during a predetermined time period. It has been found experimentally that minute ventilation (MV) tracks very well with metabolic need over a range of heart rates and, therefore, will provide a good index for a rate adaptive pacemaker, provided that the technique used to derive the MV signal is not contaminated by events other than respiratory activity.
It is known in the art that impedance plethysmography may be used to obtain an impedance vs. time analog signal that is modulated in accordance with the expansion and contraction of the thoracic cavity during inspiration and expiration. As is explained in the Pederson et al. Pat. No. 5,137,019, assigned to applicant's assignee, the impedance vs. time signal includes not only variations due to respiratory activity but due to cardiac activity and body motion as well. It is, therefore, desirable to isolate the information in the waveform relating to respiratory activity from that due to other causes. In the Hauck U.S. Pat. No. 5,074,303 entitled "Rate Adaptive Cardiac Pacer Incorporating Switched Capacitor Filter with Cut-Off Frequency Determined by Heart Rate", there is described an arrangement in which either systolic events or respiratory events can be selectively enhanced or attenuated by adjusting the analog filter parameters as a function of sensed heart rate. The present invention provides an alternative way for implementing the general concept disclosed in the Hauck Pat. No. 5,074,303 for effectively filtering out components in the Z vs. T waveform due to systolic function and motion artifacts over a range of heart rates and breathing rates which the patient would be expected to encounter in going from a sedentary state to a condition of vigorous exercise.